Chemical-mechanical polishing (CMP) is a well-known technique for removing materials on a semiconductor wafer using a polishing device and a polishing agent. The mechanical movement of the polishing device relative to the wafer in combination with the chemical reaction of the polishing agent provide an abrasive force with chemical erosion to planarize the exposed surface of the wafer or a layer formed on the wafer. Rotating, orbital, and linear polishers are three types of tools that can be used in the CMP process. With a rotating polisher, a rotating wafer holder supports a wafer, and a polishing pad on a moving platen rotates relative to the wafer surface. In contrast, the platen of an orbital polisher orbits as opposed to rotates during polishing. With a linear polisher, a flexible belt moves a polishing pad linearly across a wafer surface, providing a more uniform velocity profile across the surface of the wafer as compared to rotating or orbital polishers.
CMP polishers can incorporate various in-situ monitoring techniques to monitor the polished surface of the wafer to determine the end point of the polishing process. U.S. Pat. No. 5,433,651 and European Patent Application No. EP 0 738 561 A1 describe rotating polishers that are designed for in-situ monitoring. In the '651 patent, a rotating polishing platen has a fixed window, which is flush with the platen but not with the polishing pad on the platen. As the platen rotates, the window passes over an in-situ monitor, which takes a reflectance measurement indicative of the end point of the polishing process. Because the top surface of the window is below the top surface of the polishing pad, polishing agent collects in the recess above the window, adversely affecting the measurement by scattering light traveling through the window.
European Patent Application No. EP 0 738 561 A1 discloses a rotating polishing platen with a fixed window, which, unlike the one in the '651 patent, is substantially flush with or formed from the polishing pad. Because the top surface of the window is in the same plane as the top surface of the polishing pad during the entire polishing process, the optical transparency of the window can be damaged when the wafer slides over the window and when pad conditioners cut small groves across the polishing pad. Since the window is not replaceable, once the window is damaged, the entire pad-window polishing device must be replaced even if the polishing pad itself does not need to be replaced.
There is a need, therefore, for an improved wafer polishing device that will overcome the problems described above.